ABSTRACT Patients with Sjgren's syndrome (SjS) or radiation therapy for head and neck cancer suffer from severe dryness as a result of salivary acinar cell loss. To restore secretory function, reprogramming of stem/progenitor cells into salivary epithelial precursors (SEP) is a critical step to take. This can be achieved by exposing stem cells to external inducing factors and/or expressing key transcription factors (TFs) in stem cells. Despite much progress in cell replacement therapy, the clinical feasibility and efficacy of bone marrow-derived mesenchymal stem cells (BM-MSCs) in salivary gland (SG) regeneration remains largely unexplored. Surprisingly, information on critical salivary TFs involving MSC reprogramming is completely absent. To fill this scientific gap, we investigated that BM-MSCs were able to differentiate into SEP in the presence of primary SG cells without cell-to-cell contact. Subsequent proteomics analyses of 2-dimensional gel electrophoresis (2-DE) and Isobaric Tag for Relative and Absolute Quantification (iTRAQ) on differentiated MSCs identified putative TFs for the first time, namely TCF3, HMG20B, ANKRD56, PTF1?, SGN1, and MIST1. Therefore, in this application, we aim to define a critical set of TFs in MSC-to-SEP conversion and prove the impact of forced expression of TFs on MSC reprogramming in unfavorable SG microenvironment caused by radiation/inflammation. Our central novel hypothesis is that expression of key TFs in stem cells prior to transplantation facilitates in vivo stem cell differentiation into saliva secreting cells in severely damaged salivary glands. Two Specific Aims include: Aim 1. Determine a critical set of TFs that drives BM- MSC differentiation into salivary precursors. We will test our hypothesis that ?TCF3, PTF1?, and MIST1 are the minimally required key TFs for MSC-to-SEP conversion.? We will test various combinations of our selected TFs for MSC reprogramming, which will involve MSC transduction with lentiviral vector(LV) expressing TFs of interest, and monitor the cells by LV-promoter-reporters and functional assays. Aim 2. Investigate the impact of TF-directed MSC differentiation on restoring saliva secretion. We will test the hypothesis that ?Forced TF expression in MSCs is critical in final differentiation of SEP in vivo and in restoring saliva flow when the SG is substantially damaged.? Our SjS mouse model with the early and late stages of SjS will be transplanted with nave MSCs or LV-TF- transduced MSCs and characterized for saliva flow. This approach is particularly invaluable since it simulates clinical situations where SGs are severely damaged beyond the ability to self-repair. Successful completion of this project should significantly advance our understanding of the critical roles of these unexplored TFs in lineage determination, MSC differentiation, and SG regeneration.